The fight-or-flight response, also called the acute stress response, was first described by Walter Cannon in 1927. His theory states that animals react to threats with a general discharge of the sympathetic nervous system, priming the animal for fighting or fleeing. This response was later recognized as the first stage of a general adaptation syndrome that regulates stress responses among vertebrates and other organisms.

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Normally, when a person is in a serene, unstimulated state, the "firing" of neurons in the locus ceruleus is minimal. A novel stimulus (which could include a perception of danger or an environmental stressor signal such as elevated sound levels or over-illumination), once perceived, is relayed from the sensory cortex of the brain through the thalamus to the brain stem. That route of signaling increases the rate of noradrenergic activity in the locus ceruleus, and the person becomes alert and attentive to the environment. Similarly, an abundance of catecholamines at neuroreceptor sites facilitates reliance on spontaneous or intuitive behaviors often related to combat or escape[How to reference and link to summary or text].

If a stimulus is perceived as a threat, a more intense and prolonged discharge of the locus ceruleus activates the sympathetic division of the autonomic nervous system (Thase & Howland, 1995). This activation is associated with specific physiological actions in the system, both directly and indirectly through the release of epinephrine (adrenaline) and to a lesser extent norepinephrine from the medulla of the adrenal glands.

A typical example of the stress response is a grazing zebra, calmly maintaining homeostasis. If the zebra sees a lion closing in for the kill, the stress response is activated. The escape requires intense muscular effort, supported by all of the body’s systems. The sympathetic nervous system’s activation provides for these needs. A similar example involving fight is of a cat about to be attacked by a dog. The cat shows accelerated heartbeat, piloerection (hair standing on end, normally for conservation of heat), and pupil dilation, all signs of sympathetic arousal.

Though Cannon, who first proposed the idea of fight-or-flight, provided considerable evidence of these responses in various animals, it subsequently became apparent that his theory of response was too simplistic. Animals respond to threats in many ways, not only by fighting and fleeing. Rats, for instance, try to escape when threatened, but will fight when cornered. Some animals stand perfectly still so that predators will not see them. Others have more exotic self-protection methods. Some species of fish change color swiftly, to camouflage themselves. Although these responses are triggered by the sympathetic nervous system, they do not fit the simple model of fight or flight. The only thing that would generate the fight method in these animals would be the law of self-preservation. This means that if the animal was attacked while camouflaged, it would instinctively counter-attack its assailant.

Furthermore, it is relatively rare that a threat from another animal results immediately in fight or flight. Usually there is a period of heightened awareness, during which each animal interprets behavioral signals from the other. Signs such as paling, piloerection, immobility, sounds, and body language communicate the status and intentions of each animal. There may be a negotiation of sorts, after which fight or flight may ensue, but which might also result in displacement behavior or playing, mating, or nothing at all. An example of this is kittens playing: each kitten shows the signs of sympathetic arousal, but they are aware of each other’s intent not to inflict real damage.

In prehistoric times when the fight or flight response evolved in humans, fight was manifested in aggressive, combative behavior and flight was manifested by fleeing potentially threatening situations, such as being confronted by a predator. In current times, these responses persist, but fight and flight responses have assumed a wider range of behaviors. For example, the fight response may be manifested in angry, argumentative behavior, and the flight response may be manifested through social withdrawal, substance abuse, and even television viewing (Friedman & Silver 2007).

Behaviorally, the fight or flight response describes men’s reactions to stressful situations better than women’s. That is, men are more likely to cope with stress via social withdrawal, substance abuse, and aggression. Some researchers believe that these aspects of the fight or flight response in men contribute to their earlier mortality, relative to women. Women are more likely to cope with stress through social support, that is, by turning to others to both give and receive instrumental and emotional aid. This pattern of responding has been called “tend and befriend,” and refers to the fact that during stressful times, women are especially likely to show protective responses toward their offspring and affiliate with others for shared social responses to threat.

Although the emergency measure of the stress response is undoubtedly both vital and valuable, it can also be disruptive and damaging. In most modern situations, humans rarely encounter emergencies that require physical effort, yet our biology still provides for them. Thus we may find our stress response activated in situations where physical action is inappropriate or even illegal. This activation takes a toll on both our bodies and our minds.